EP2803960A1 - Procédé d'analyse de signaux et spectromètre à réflectométrie temporelle THz équipé d'un émetteur-récepteur pour son application - Google Patents

Procédé d'analyse de signaux et spectromètre à réflectométrie temporelle THz équipé d'un émetteur-récepteur pour son application Download PDF

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EP2803960A1
EP2803960A1 EP13167488.9A EP13167488A EP2803960A1 EP 2803960 A1 EP2803960 A1 EP 2803960A1 EP 13167488 A EP13167488 A EP 13167488A EP 2803960 A1 EP2803960 A1 EP 2803960A1
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Prior art keywords
thz
modulation
pulse
pulses
frequency
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German (de)
English (en)
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Stefan Busch
Martin Koch
Thorsten Probst
Michael Schwerdtfeger
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Philipps Universitaet Marburg
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Philipps Universitaet Marburg
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/0232—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using shutters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
    • G01J3/433—Modulation spectrometry; Derivative spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
    • G01N21/3586—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation by Terahertz time domain spectroscopy [THz-TDS]

Definitions

  • the invention relates to a method for signal analysis for THz time domain spectroscopy (Terahertz Time Domain Spectroscopy - THz-TDS) and equipped with a transceiver THz time domain spectrometer with 0 ° reflection geometry, which is suitable for the application of this method.
  • transceiver is a combination of transmitter (transmitter, source) and receiver (receiver, detector). It refers to an assembly in which the functions send and receive are combined.
  • transmitter transmitter
  • receiver receiver
  • the definitions of a transceiver used in the literature are not clear, so a definition must be made here.
  • a transceiver is understood to mean an assembly which has only a single antenna, which functions both as a transmitter and as a receiver.
  • assemblies which have two antennas are also referred to in the literature as transceivers if both antennas are arranged directly adjacent to one another and thus can be implemented in the form of a compact probe.
  • Such two antenna arrangements are hereinafter referred to as pseudo transceivers in this application because they do not conform to the above definition of a transceiver (one antenna only).
  • transceiver and pseudo-transceiver can in principle be used in any frequency range of the electromagnetic spectrum and can also be transferred to mechanical waves (eg ultrasound).
  • mechanical waves eg ultrasound
  • the THz spectroscopy, the transceivers and pseudo transceivers described below are always those which are in the THz range (defined here as the frequency range between 30 GHz and 10 THz) or at least in one subrange the same are used.
  • the signal analysis method according to the invention can also be used in a frequency range which is extended beyond this range.
  • both transceivers and pseudo-transceivers have a modulator, an electronic circuit and the necessary electrical connections.
  • An indispensable component of the electronic circuit for transceivers is a narrowband filter, preferably a lock-in amplifier. Pseudo transceivers do not necessarily have such a filter, but in order to improve the signal quality they are also usually equipped with it.
  • transceivers described in this patent application are always transceivers suitable for the THz range (THz transceivers). However, they are always referred to below as a transceiver.
  • An application in industrial processes is z. For example, quality control of raw materials whose consistent composition can be verified by THz spectroscopy.
  • THz radiation is used to differentiate between diseased and healthy tissue, with endoscopic examinations being particularly desirable.
  • THz radiation strong absorption in water, strong reflection on metallic surfaces
  • measurement arrangements with reflection geometry must be used.
  • These should be designed so small and compact that their physical dimensions do not limit their use in process control.
  • the probes for the emission and detection of THz radiation should be designed to be light and largely free-moving, allowing samples to be taken at various locations and endoscopic examinations.
  • the dimensions of the measuring arrangements are determined primarily by the distance between THz transmitter and THz receiver, it is desirable to combine THz transmitter and THz receiver in a confined space in a probe and ideally even transmitter and receiver function with a single , Accommodated in the antenna to realize antenna, so perform the probe according to the above definition as a transceiver.
  • the associated measuring arrangement is characterized by a 0 ° reflection geometry in which the emitted and the detected THz signal move in opposite directions on the same optical path.
  • the omission of the second antenna also makes it possible to achieve a considerable reduction in system costs.
  • THz signal directed to the sample to be examined and the detection of the much weaker, THz signal reflected by the sample at the same location (at the antenna forming the transceiver), so that both THz signals overlap and have to be separated reliably.
  • This signal analysis requires appropriate signal modulation techniques and associated modulators.
  • a transceiver for the THz frequency range is disclosed in the patent US 6844552B2 (Zhang et al., Issued Jan. 18, 2005 ).
  • the assembly comprises the prior art assemblies of a THz spectrometer: a femtosecond laser (fs laser) providing a train of short infrared pulses, and a delay unit beam splitter which receives the infrared pulses provided by the fs laser, respectively divides into an excitation pulse and an interrogation pulse and delays the interrogation pulse by a defined, tunable time span with respect to the stimulation pulse.
  • fs laser femtosecond laser
  • a delay unit beam splitter which receives the infrared pulses provided by the fs laser, respectively divides into an excitation pulse and an interrogation pulse and delays the interrogation pulse by a defined, tunable time span with respect to the stimulation pulse.
  • both pulse sequences hit a transceiver, z. B.
  • a connected to a bias DC voltage source photoconductive antenna that emits a THz field when acting on a pulse and detects when acting on the associated, time-delayed interrogation pulse the (eg, on a sample) reflected, instantaneous THz field.
  • the THz field undergoes amplitude modulation by means of a mechanical modulator (chopper) arranged in the THz beam path, the maximum attainable modulation frequency being in the range of 1-2 kHz.
  • the modulation frequencies achievable with the mechanical modulator are limited to maximum values in the range of a few kHz, which leads to a low measuring speed.
  • alternative methods are needed in which the signal modulation does not take place in the THz beam path.
  • YB Ji et al. (Optics Express 17 (2009) 17082 ) describe a suitable as transmitter and receiver for THz radiation probe that overcomes the disadvantages mentioned and is particularly suitable for endoscopy.
  • the probe has two directly adjacent photoconductive antennas, which act as a transmitter (generator) or receiver (detector).
  • the antennas are housed in housings with a cross section of 4 mm x 6 mm each, which are connected to each other on the longer side, so that the endoscope receives a cross section of 8 mm x 6 mm. Both antennas are each connected via an optical fiber to a beam guiding system.
  • the modulation takes place in this solution by applying an AC voltage (90 V ac , 500 Hz) to the transmitter antenna.
  • an electronic modulation method is used in which the amplitude of the sequence of the excitation pulses is modulated with a selected modulation frequency.
  • a signal modulating method suitable for such a reflection spectrometer with a transceiver would allow 0 ° reflection measurements (vertical incidence) with a miniaturized, almost freely moving THz probe and would thus be ideally suited for inline process control in hard-to-reach locations and for endoscopic examinations.
  • the object of the invention is therefore to provide a signal modulation method for THz measuring devices, which makes it possible to increase their sensitivity significantly and thus to be able to analyze even extremely weak THz signals still quantitatively. In particular, it should be possible to extract a THz signal from a several orders of magnitude higher fundamental signal.
  • the signal modulation method is intended to be particularly applicable to a transceiver-equipped THz time domain spectrometer with 0 ° reflection geometry (i.e., perpendicular beam incidence) in which emitted and reflected THz beams move in opposite directions on the same path.
  • a further object of the invention is to provide such a THz time domain spectrometer equipped with a transceiver with 0 ° reflection geometry, which uses the signal modulation method according to the invention in signal analysis.
  • the object is achieved by the signal analysis method comprising a novel signal modulation method according to claims 1 and 2. Furthermore, the object is achieved by the provision of a modulation system according to claims 3 to 5 and a equipped with a transceiver THz time domain spectrometer with 0 ° reflection geometry according to claims 6 to 12 (hereinafter short THz spectrometer), which is suitable for the application of the signal modulation method according to the invention.
  • a modulation system according to claims 3 to 5 and a equipped with a transceiver THz time domain spectrometer with 0 ° reflection geometry according to claims 6 to 12 (hereinafter short THz spectrometer), which is suitable for the application of the signal modulation method according to the invention.
  • An essential feature of the signal modulation method according to the invention is that the signal modulation takes place in the optical part of the THz spectrometer, whereby both the sequence of the excitation pulses and the sequence of the interrogation pulses are modulated with different modulation frequencies.
  • An essential feature of the signal analysis method is that the THz signal to be measured occurs at the mixing frequencies generated from the two modulation frequencies and thus can be reliably extracted from the total signal in the frequency domain.
  • a fs laser operating at a repetition rate f rep of typically 70 MHz to 250 MHz supplies the THz time domain spectrometer with a series of extremely short laser pulses whose wavelength is in the range of 600 nm to 1600 nm, preferably in the near IR range of 780 nm to 1550 nm.
  • the pulse duration ⁇ T is in the range of 1 fs to 1 ps, preferably in the range 30 fs to 100 fs.
  • Each pulse of this pulse sequence is divided by means of a beam splitter into a starting pulse and a polling pulse, so that a sequence of excitation pulses and a sequence of interrogation pulses arise.
  • One of the pulse sequences is conducted over a delay path, in which by changing the optical path length a tunable in the range of a few femtoseconds to nanoseconds time delay between the two pulse sequences is realized.
  • Each pulse of the sequence of excitation pulses is thus assigned exactly one pulse of the sequence of the interrogation pulses. Two such slightly time-shifted pulses form a pair of pulses, which is separated from the next pair of pulses by a much longer period of time (order of magnitude 10 ns).
  • the THz time domain spectrometer is equipped with a THz emitter and a THz detector, which are spatially separated.
  • the path lengths of the two pulse sequences in the spectrometer are selected so that initially always the excitation pulse reaches the transmitter antenna acting as a THz emitter.
  • the associated interrogation pulse reaches the receiver antenna acting as a THz detector only after a defined, tunable delay time.
  • Essential is the different wiring of the two photoconductive antennas.
  • an external bias DC voltage U 0 whose value is typically selected between 1 V and 100 V.
  • U 0 an external bias DC voltage
  • the receiver antenna is no bias voltage, instead, it is connected to a sensitive device for measuring current, preferably a transimpedance amplifier with a downstream lock-in amplifier connected.
  • Both antennas are basically inactive, only by the action of one of the extremely short, intense laser pulses charge carriers are generated and the antenna function is switched on for a short time.
  • free charge carriers electrons and, due to their low mobility, negligible holes
  • the stored energy W is converted into kinetic energy of the electrons and THz radiation.
  • the capacitor with the capacitance C Due to the briefly extremely high carrier density due to the short, intense laser pulse, the capacitor with the capacitance C is discharged completely abruptly.
  • a semiconductor material with an extremely short charge carrier lifetime is selected (eg low-temperature-grown GaAs). These boundary conditions cause the charge generated by the accelerated photoelectrically generated charge carriers caused current pulse has a very short duration of the order of a few ps.
  • the THz pulse emitted in accordance with Maxwell's equations also has this short duration and therefore a very high bandwidth.
  • the amount of its field strength E THz is proportional to the temporal change of the current density j (t) of the electrons: e THz t ⁇ ⁇ j t ⁇ t ,
  • the maximum values of the current density j (t) and thus the field strength E THz of the emitted THz pulse are determined by the bias voltage U 0 , the optical power of the fs laser, the semiconductor material and the antenna structure.
  • the sequence of time-delayed sampling pulses is directed to the receiver antenna.
  • Accurate adaptation of the path lengths in the spectrometer ensures that, simultaneously with the arrival of a polling pulse, the associated THz pulse emitted by the transmitter antenna, which already passes through the measurement path and thereby penetrates (transmits) the sample (or reflects on the sample surface was) arrives at the receiver antenna.
  • the THz pulse contains the spectral information about the sample, which is now to be extracted.
  • the receiver antenna to which no bias voltage is applied, is connected to a sensitive device for current measurement, preferably a transimpedance amplifier with a downstream lock-in amplifier.
  • the sample pulse delivered by the fs laser is short compared to the duration of the THz pulse of several ps.
  • the interrogation pulse switches the antenna on for a few femtoseconds, so that the instantaneous electric field of the THz pulse accelerates charge carriers during this period and induces a current flow in the antenna. Thus, only a short portion of the THz pulse is measured with a polling pulse.
  • the charge carriers generated by the interrogation pulse follow the instantaneous electric field of the THz pulse and provide an electrical current signal which is generated by means of the device arranged between the antenna contacts is measured for current measurement. Since no bias voltage, so no external electric field, applied, this current signal is largely determined by the incoming THz field.
  • a low-pass characteristic circuit connected to the receiver antenna causes an averaged DC signal to be measured instead of a signal having the repetition rate f rep of the laser pulses.
  • the delay time ⁇ t is tuned and determined in this way a current reading for each position ⁇ t of the THz pulse in the time domain, the waveform of the complete THz pulse is thus sampled (sampling method).
  • L gap is the width of the gap of the antenna (ie, the pitch of the electrode tips)
  • s is the thickness of the semiconductor substrate
  • d is the width of the electrode tips forming the gap of the antenna.
  • ⁇ is a conversion factor that describes the relationship between the laser power and the number of photoelectrically generated charge carriers.
  • the devices described in the prior art allow the flow of current I ( ⁇ T) to be measured.
  • the measured direct current signal is preferably modulated with a frequency in the range of a few 10 Hz up to a few kHz (with mechanical modulation) and up to a few MHz (with electro-optical modulation) and by means of lock-in technology evaluated.
  • a modulation method which allows mechanical or electro-optical modulation with high modulation frequencies in the optical branch of a THz time domain spectrometer and is particularly suitable for use in THz time domain spectrometers with 0 ° reflection geometry, which are equipped with a miniaturized transceiver arrangement ,
  • a transceiver is a single photoconductive antenna that acts both as a transmitter antenna and as a receiver antenna. Pickup and interrogation pulses alternate with the antenna and switch on the antenna function for a short time.
  • the transceiver In order for the transceiver to function as a transmitter antenna, it must be assigned a bias voltage U 0 which, when an exciting pulse impinges, causes a unipolar current pulse in the mA range which, as described above, corresponds to the emission of a THz frequency according to Maxwell's equations. Pulse is connected.
  • This bias DC voltage is in the range of 1 V to 100 V, preferably in the range 10 V to 40 V. It also inevitably affects when a query pulse, so here, too, a unipolar current pulse in the mA range, which is one weaker by several orders of magnitude , usually superimposed in the nA range component superimposed. This component is generated by the instantaneous field strength of the returning THz pulse containing the desired measurement information.
  • the measurable current I at the transceiver therefore consists of three components: the two unipolar components I S and I E caused by the excitation and interrogation pulse as well as the much weaker component I THz caused by the incoming THz field, which coincide with I E occurs.
  • the excitation and interrogation pulses are always separated by a much longer period of time of several tens of picoseconds, so that the currents produced by them do not influence each other.
  • the total current I thus results directly from the sum of the partial currents, wherein the simultaneously occurring components I E and I THZ are separated in time by the component I S.
  • This object is achieved by modulating the amplitudes of the sequence of the excitation pulses and the sequence of the interrogation pulses with different frequencies. Only methods in which one of these pulse sequences is modulated are known from the prior art. The advantageous analysis possibilities resulting from the method according to the invention of modulating the amplitudes of both pulse sequences with different frequencies are clear as a result of the following derivation, which quantitatively describes the effect of the modulation method on the three current components. For the sake of simplicity, a sinusoidal amplitude modulation is used below (harmonic modulation). However, the derivative can be generalized to any uni- or bipolar modulation functions (eg, rectangular, triangular and sawtooth modulation). These are thus also suitable for the modulation method.
  • the current flow I at the transceiver is composed of the components I S , I E and I THz .
  • the electrical resistance of the photoconductive antenna is always so high that no measurable current flow occurs. Only during the impingement of an excitation or interrogation pulse, the resistance is reduced to low values R S and R E , which allow a measurable current flow when applying a bias voltage to the antenna or an incoming THz field.
  • the amplitude and thus the power of the sequence of the excitation pulses with a first modulation frequency f S and the amplitude and thus the power of the sequence of the interrogation pulses are modulated with a second, different modulation frequency f E :
  • P S laser P S . 0 laser ⁇ sin 2 ⁇ ⁇ ⁇ f S ⁇ t .
  • P e laser P e . 0 laser ⁇ sin 2 ⁇ ⁇ ⁇ f e ⁇ t ,
  • the modulation frequencies f S and f E must be smaller than the repetition rate f rep of the fs laser. It is important here that the frequencies f S and f E are not suppressed by the low-pass characteristic of the antenna.
  • R S and R E The modulation of the power of the laser pulses causes a modulation of R S and R E :
  • R S t L gap 2 s ⁇ ⁇ ⁇ P S . 0 laser ⁇ sin 2 ⁇ ⁇ ⁇ f S ⁇ t
  • R e t L gap 2 s ⁇ ⁇ ⁇ P e . 0 laser ⁇ sin 2 ⁇ ⁇ ⁇ f e ⁇ t
  • the THz field E THz reflected on the specimen containing the desired THz spectral spectral information has an amplitude E THz, 0 in the range of 0.01 V / cm to 10 V / cm when incident on the antenna.
  • E THz amplitude
  • L dipole length
  • the measurable at the antenna component I THz of the photocurrent is thus determined by two overlapping modulation functions:
  • the modulation of U THz, 0 is carried out at the frequency f S , which of R E, however, with the frequency f E.
  • the modulation method according to the invention thus effects a separation of the current component I THz ( t ) containing the THz signal from the components I S ( t ) and I. E ( t ) of the fundamental currents.
  • f S and f E must be chosen only so that the sum and difference frequencies have a sufficiently large distance from the fundamental frequencies, which, how Fig. 1 shows is easily possible.
  • both sub-components have an identical information content, it is sufficient also one of the two sub-components (f at the differential frequency S - f E or at the sum frequency f S + f E) to measure in order to extract the desired spectral THz information.
  • This measurement can easily be carried out by means of a frequency filter, which is designed as a narrow-band filter, preferably as a lock-in amplifier, by f S - f E or f S + f E as the center frequency of the filter or as the reference frequency for controlling the lock -in amplifier can be used.
  • the modulation methods known from the prior art do not permit such a separation of the THz signal.
  • these methods only one pulse train is modulated, z. B. the sequence of excitation pulses with the modulation frequency f S.
  • I THz is thus superimposed in the frequency range of the component I s, which is higher by several orders of magnitude, and therefore can not be measured.
  • the new modulation method makes it possible to reliably extract a very small current component I THz from two much stronger fundamental currents I S , I E and to measure it exactly.
  • this method it is possible for the first time to realize a transceiver in which the modulation is not in the THz beam path but already in the optical part of the THz spectrometer (in the spatially separated optical paths of the sequences of the excitation and interrogation pulses) both pulse sequences are brought together again in an optical fiber or in the free jet).
  • the modulation method according to the invention is not limited to harmonic (sinusoidal) modulation functions. Rather, any modulation functions can be used. Particularly suitable are a harmonic modulation with constant offset and a modulation with unipolar rectangular pulses. However, triangular and sawtooth modulation as well as combinations thereof and any other shapes are also applicable.
  • Modulation frequencies f S , f E between 50 Hz and 50 MHz are suitable; they are preferably in the frequency range 100 Hz ⁇ f S , f E ⁇ 1 MHz, particularly preferably in the frequency range 1 kHz ⁇ f S , f E ⁇ 100 kHz.
  • the amplitude modulation is carried out mechanically or electro-optically, with electro-optically fundamentally higher modulation frequencies being achievable.
  • the modulation frequencies are selected in the lower range (50 Hz-2 kHz, preferably 100 Hz-1 kHz), while electro-optical modulation in the upper range (1 kHz-50 MHz, preferably 10 kHz-1 MHz) of said intervals ,
  • high modulation frequencies are to be preferred. However, they must remain lower than the repetition rate f rep of the fs laser, which lies within the range of 100 MHz, since at each sampling position the mean value is to be formed from a large number of individual measurements. Due to the transition from the mechanical amplitude modulation (typical modulation frequency 1 kHz) to the electro-optical amplitude modulation (typical modulation frequency 100 kHz), the measuring speed can be increased by a factor in the region of 100. If very high modulation frequencies ( f S , f E ⁇ 10 MHz) are selected, it must be ensured that f S , f E are less than half the repetition rate f rep of the fs laser.
  • the wavelength of the fs laser is in the conventional interval according to the prior art 600 nm - 1600 nm, preferably the wavelengths 800 nm, 1064 nm, 1310 nm and 1550 nm and particularly preferably the wavelength 1550 nm are used.
  • the repetition rate of the fs laser is preferably in the usual 70 MHz - 250 MHz according to the prior art, but repetition rates f rep in the extended interval 1 kHz - 2 GHz can also be used. Very small repetition rates (from 1 kHz) are suitable for mechanical modulation with very low modulation frequencies ( ⁇ 100 Hz), very high repetition rates (up to 2 GHz) for electro-optical modulation with very high modulation frequencies ( ⁇ 10 MHz).
  • the pulse durations of the excitation and interrogation pulses are between 1 fs and 1 ps, preferably between 30 fs and 100 fs, the delay time is between 0 fs and 20 ns, preferably between 50 ps and 500 ps.
  • the pulse sequences generated by the beam splitter from the output pulse train 21 pass through the optical branch of the spectrometer completely in the free jet, completely in optical fibers (preferably glass fibers) or in a combination of free jet and optical fibers , With the help of a suitable optical component, both pulse sequences are brought together again, which can also be done both in the free jet and in an optical fiber.
  • This combination is also known from the cited in the prior art US 6844552B2 already known.
  • the inventive transceiver-equipped THz time domain spectrometer with 0 ° reflection geometry differs from the known THz spectrometers in that it has a modulation system 50, which pass through both pulse trains 21 a and 21 b, before they are merged again.
  • the modulation system 50 subjects both pulse trains 21 a and 21 b amplitude modulation according to the method described above, thereby varying in different modulated pulse trains 21 a 'and 21 b' are transferred, which are then merged in the manner described above.
  • modulators for the modulation system 50 which allow the modulation method according to the invention to be applied to the two pulse sequences 21 a and 21 b when they are guided in free-jet or when the two pulse sequences are in separate optical fibers 62 a and 62b to be used as a fiber-coupled modulation method in the area of these fibers.
  • the subsequent merger of the modulated pulse trains 21 a 'and 21 b' is by means of a beam splitter (in beam guidance in the free jet) or a fiber optic coupler 63, z. B. a 3-dB coupler, (with fiber-coupled beam guidance) possible.
  • the fiber optic coupler 63 combines both pulse sequences in an optical fiber 62. If a combination of beam guidance in the free jet and fiber-coupled beam guidance is selected, coupling elements 61 a and 61 b are required for the transition from free jet to optical fiber.
  • Modulators suitable for use in the modulation system 50 will be described below. Modulators known from the prior art can be used, but since the modulation method according to the invention requires two different modulation frequencies in different optical paths, they must be adapted accordingly. For this purpose, on the one hand, it is possible to modify a conventional modulator so as to provide two modulation frequencies differing by a suitably chosen factor, or to drive two conventional modulators with a common reference frequency, one of the modulators providing the reference frequency as the modulation frequency, while the second modulator provides a different modulation frequency that is generated by multiplying the reference frequency by a suitably chosen factor. Corresponding embodiments are described below.
  • choppers Mechanical modulators, commonly referred to as choppers, are usually designed as disk-shaped apertures (chopper wheels) with equidistantly arranged slots on a circular ring.
  • Commercially available choppers usually have a number of exchangeable chopper wheels with different numbers of slots, which are made to rotate with adjustable frequencies from a few Hz up to about 100 Hz.
  • the Chopper 218M from Bentham has four exchangeable chopper wheels with 2, 5, 10 and 30 slots and allows to modulate a free jet impinging on a rotating chopper wheel with frequencies between 2.5 Hz and 3 kHz.
  • this chopper can deliver only a single modulation frequency at any one time.
  • the modulation system 50 is preferably implemented with only one mechanical modulator which provides two modulation functions with different modulation frequencies, which are in a suitable fixed ratio f S / f E.
  • This is z. B. achievable by a suitably designed chopper 51, as it is in Fig. 2 included modulation system 50 has.
  • the chopper wheel 51 has two concentric circular rings with a different number of slots, which corresponds to the desired ratio f S / f E.
  • the modulation functions provided by such a chopper wheel are thus coupled in a phase-locked manner.
  • the form of the modulation functions depends on the beam shaping of the incident light beams. If the incident laser beams are focused at a point in the chopper plane, they are abruptly switched on and off by the chopper, resulting in a rectangular modulation. If, on the other hand, they run as collimated laser beams with a beam diameter of a few millimeters through the chopper plane, then modulation functions with trapezoidal or triangular progression as well as (almost) harmonic modulation functions with constant offset can also be realized.
  • the selection of the chopper wheel and the adjustment of its rotational frequency (and thus of the modulation frequencies) takes place with the aid of a modulator controller, which is part of the measuring and control electronics of the spectrometer according to the invention.
  • electro-optic modulators An alternative to mechanical modulators are electro-optic modulators. They can be used both in free-jet and integrated into optical fibers and allow much higher modulation frequencies.
  • a modulation system 50 with electro-optical modulators in the free jet (at the position of the chopper wheel 51) or as a fiber-coupled modulation system in the portion of the connecting fibers 62a and 62b of the fiber optic coupler is used.
  • Executable is such a modulation system z. B. in the form of two electro-optical switches, which are discrete assemblies in the free-jet areas of the two Pulse trains (consequences of the stimulation and interrogation pulses) are arranged or, in a preferred embodiment, are integrated as a fiber-coupled electro-optical switch in the two connecting fibers of the fiber optic coupler.
  • electro-optical switches suitable VOAs (Variable Optical Attenuators) are used, which allow switching times down to a few nanoseconds. The required modulation frequencies, which are at a maximum of 50 MHz, are therefore easily accessible.
  • an electro-optic modulation system is also connected to a modulator controller, with the aid of which the two electro-optical switches are set to different modulation frequencies and functions, which then differently modulate the two pulse trains passing through the modulation system.
  • the output of the fiber optic coupler 63 is connected to the transceiver 70 via an optical fiber 62 which directs the merged bursts 21a 'and 21b' to the photoconductive excitation location of the antenna of the transceiver.
  • the coupling via a flexible optical fiber and the very compact, small and lightweight probe make it possible to perform the probe almost freely. In particular, measurements in hard to reach cavities, eg. As in the final inspection of industrial products or in endoscopic examinations in medicine easily possible. Particularly advantageous is the absence of any modules between the transceiver 70 and the sample to be examined 80, so that the transceiver of the sample to be examined is approaching or even bring into direct contact with this. This possibility clarifies Fig.
  • transceiver 70 is shown in an encapsulated form (probe) with a hemispherical THz lens 72.
  • the miniaturized photoconductive antenna housed in the probe is shown in FIG Fig. 2 not visible.
  • the short distance between the transceiver and the sample makes it possible to reduce the required transmit and receive power. Furthermore, unwanted influences due to the atmospheric absorption in the THz beam path are reduced or completely eliminated.
  • Strip conductors 74 and antenna tips 75 are metallic, preferably made of copper, silver or gold. Between the antenna tips 75 with the distance L gap is the photoconductive excitation site 76 of the antenna.
  • the dimension of the antenna is determined essentially by the length and the spacing of the strip conductors (dipole length L), which are typically ⁇ 4 mm and ⁇ 500 ⁇ m.
  • the antenna can thus be accommodated on a substrate whose diagonal dimension is less than 5 mm. Since the transceiver has only a single antenna, it can thus be implemented in miniaturized form as a probe with a rectangular cross section (edge lengths ⁇ 5 mm) or a circular cross section (diameter ⁇ 5 mm).
  • the circular cross-section is for certain applications, esp.
  • the endoscopy particularly advantageous (low risk of injury), but there are of course arbitrarily shaped cross-sections, z. B. square, polygon, oval, possible.
  • the height of the probe is preferably ⁇ 8 mm. Of course, the probe can also be designed with larger dimensions.
  • the pulse 91 (with the power P S laser )
  • the antenna On the photoconductive stimulation site 76, the antenna is switched momentarily conductive, charged by the bias voltage U 0 capacitor C is discharged abruptly by the photo current I S and a portion of the electrostatically stored in C power in a decelerating THz pulse 93a which is directed by the THz lens 72 to the fixed sample 80, where it is reflected while receiving the spectral THz sample information.
  • the reflected THz pulse 93b travels back to the transceiver 70 and is focused by the THz lens 72 onto the photoconductive excitation site 76.
  • the pulse duration of the outgoing THz pulse 93a is higher by approximately 2 orders of magnitude than the pulse duration of the stimulation pulse 91 generating it.
  • the pulse duration of the reflected THz pulse 93b containing the spectral THz sample information is approximately 2 orders of magnitude higher than the pulse duration of the polling pulse 92.
  • the antenna will again for a very short period of time (substantially shorter than the duration of the incoming THz pulse) turned on, the already recharged by the bias voltage U 0 capacitor C is discharged again, so that a short-term current flow I E in mA range, which is a weak, by the instantaneous value THz Field generated at the photoconductive excitation site 76, component I THZ superimposed in the nA range.
  • the waveform of the THz pulse is composed of a plurality of instantaneous values (sampling).
  • Fig. 3c schematically shows two particularly suitable unipolar rectangular modulation functions with the modulation frequencies f S and f E.
  • Fig. 3d schematically shows how the current flow I generates at the photoconductive antenna to a proportional voltage U, which is measured by means of a load resistor R load .
  • measurement and control electronics 30 of the THz spectrometer comprises in addition to the already mentioned modulator controller 31, a voltage source 32 for supplying the photoconductive antenna of the transceiver with a constant bias voltage, a transimpedance amplifier 33 for converting the current signal measured at the transceiver I. in a proportional thereto voltage signal U, a frequency filter 34, embodied here as a lock-in amplifier, by the modulator controller 31 with a reference frequency

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EP13167488.9A 2013-05-13 2013-05-13 Procédé d'analyse de signaux et spectromètre à réflectométrie temporelle THz équipé d'un émetteur-récepteur pour son application Withdrawn EP2803960A1 (fr)

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EP3084376A4 (fr) * 2013-12-17 2017-08-09 Picometrix, LLC Système d'émission et de réception d'un rayonnement électromagnétique
US11469509B2 (en) 2016-09-07 2022-10-11 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Terahertz transceivers

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3084376A4 (fr) * 2013-12-17 2017-08-09 Picometrix, LLC Système d'émission et de réception d'un rayonnement électromagnétique
US9998236B2 (en) 2013-12-17 2018-06-12 Picometrix, Llc System for transmitting and receiving electromagnetic radiation
US11469509B2 (en) 2016-09-07 2022-10-11 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Terahertz transceivers

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